The invention relates to a reading device and system.
The invention can be applied especially to the reading of magnetic or optical recordings and, in this context, to the reading of recordings in recording systems such as computer peripherals and all types of professional systems.
It can be extended to recordings on optical tape and magnetic or optical disks provided that several adjacent information elements are to be read thereon in parallel.
The high-density recording of information on a recording medium raises a problem of disturbance (or cross-talk) when the information elements are very close to one another. A correction of cross-talk is then needed to optimize the performance characteristics of the system.
The French patent application No. 92 15474 (U.S. Pat. No. 5,493,553) describes a system for the correction of cross-talk in a system for the reading of multiple-track recordings. High-density recording on parallel tracks raises a twofold problem in re-reading: these relate to track following and track separation. The small width of the tracks (less than 20 .mu.m) means that it is difficult, on a tape reader, to ensure the precision of the track-following operation solely by the mechanical guidance of the edge of the tape. The need to ensure the inter-operational quality of the tapes and readers increases this difficulty.
Referring to FIG. 1 it can be seen that, in a system such as this, several tracks BD1-BDn are read simultaneously by means of a magneto-optical system TL using a linear CCD sensor TL6. The cross-talk from the first neighboring tracks of each track is computed in real time. FIG. 2 shows that the N information elements on the tracks BD1-BDn are read by the CCD sensor TL6 and received by a cross-talk corrector 1. Since the train of samples in series X corresponds to consecutive tracks BD1-BDn, it is arranged in the sequence 1, 2, . . . , N-1, N, 1, 2, . . . if the number of tracks analyzed is N. The corrector corrects the cross-talk in real time in taking account of the previously computed coefficients and of the known values of the disturbance-causing neighbors and transmits a train of corrected samples X'. This system therefore enables the cross-talk coming from the first neighboring tracks to be eliminated.
However, in this system, the linear CCD sensor, through its optical/electronic transfer characteristics, sets the overall electrical performance values and, especially, the bit rate of the device. To obtain higher information bit rates, it is therefore necessary to increase the scanning rate of the CCD.
It is an object of the invention to increase the bit rate of the system and hence increase the processing speed.
The French patent application No. 94 14147 (U.S. Pat. No. 5,703,845) describes a system for the correction of cross-talk in a reading system that uses commercially available fast CCD sensors that provide differentiated outputs for the even-parity and odd-parity photodetectors. In order to double the sampling rate, the pixels are classified in two groups depending on the parity of their rank. There are then obtained, at the output of an N-pixel CCD, two simultaneous frames formed by N/2 samples corresponding to the N/2 photodiodes of each group. However, this type of CCD sensor cannot be used to read a very large number of tracks at a high bit rate.
The present invention uses CCD sensors whose line of sensors is sub-divided and therefore has several intermediate outputs to sustain high bit rates. With each output, there is associated a set of N photodetectors, each giving an information element (or sample) that is an image of its respective track at a fixed rate. At output of the CCD, there are then obtained several trains of simultaneous sub-frames constituted by N samples corresponding to the N photodiodes of each group, hence to N adjacent tracks. The information elements coming from all the trains during a sub-frame constitute a frame. The duration of a sub-frame is actually equal to M clock cycles for it is necessary to add "inactive" cycles, the number of which is M-N. These cycles are inactive from the point of view of useful information, but are necessary to complete the discharge, out of the component, of the charges collected by the photodiodes.
The basic principle of cross-talk correction in the French patent application No. 92 15474 can be applied for a single train of samples in series corresponding to consecutive tracks and therefore ordered according to the sequence 1, 2, . . . , N-1, N, 1, 2, . . . should N tracks be analyzed. Another problem therefore arises. This is the problem of correcting cross-talk for multiple trains of signals coming from different groups of photodiodes.
For this purpose, the invention uses a cross-talk correction circuit for each train. Each circuit is essentially the same as the one already used in the system with only one frame as described in the French patent application No. 92 15474 (U.S. Pat. No. 5,493,553). Each correction circuit needs three successive samples if the cross-talk should be limited to the first neighbors. These three samples correspond to a central track and to its right-hand and left-hand neighbors. However, for samples located at the end of each sub-frame, one of the neighboring samples belongs to a different sub-frame. Although it is possible simply to refrain from correcting the end samples for cross-talk or to assign them less important information such as a reference signal, this leads to a major reduction of the bit rate in the case of short CCD "fragments" (for example 2/16 in the case of a 16-pixel CCD sensor). However, the invention corrects all the samples of a multiple-output CCD and requires a special sequencing to process the first and last samples of each sub-frame.
Preferably, this sequencing makes use of the "inactive" cycles present in a CCD frame.